FIG. 1 is a block diagram illustrating selected components of an information storage system (disk drive) 110 according to of the prior art. Disk drives have one or more rotatable disks 111 on which ferromagnetic thin materials are deposited. The disk drive includes data recording disk 111, pivoting actuator arm 113, and slider 112 that includes a read head and a write head. The functional blocks include servo system 90, read/write electronics 114, interface electronics 115, controller electronics 116, microprocessor 117, and RAM 118. A disk drive can include multiple disks stacked on a hub that is rotated by a disk motor, with a separate slider for each surface of each disk. The term servo wedge 120 is used to mean the contiguous set of servo fields extending from ID to OD on the disk.
Disk 111 will typically have multiple servo wedges 120 arranged radially around the disk, but only two are shown for simplicity. Information recorded on the disks is generally organized in concentric tracks or, alternatively, the tracks can be arrange in a plurality of spiral tracks. (For a description of spiral tracks see, for example, U.S. Pat. No. 7,113,362 to Lee, et al. Sep. 26, 2006.) In embodiments either of these tracks organizations can be used, and the term “tracks” will be used generically to include these any other similar forms of arrangement.
Typically as part of the manufacturing process permanent servo information is recorded on the disks that provides information to the system about the position of the heads in the slider when the disks are rotating during operation. The Servo Identifier (SID) data (in the servo wedges) on the disk provides several fundamental functions and is conventionally arranged in four distinct fields in each of the plurality of servo sectors angularly spaced around the disk. First, the servo data supplies a timing mark (known as the Servo Track Mark (STM) or equivalently Servo Address Mark (SAM)) which is used to synchronize data within the servo fields, and also provides timing information for write and read operations in the user data portions of the track. Second, the SID supplies a 10-30 bit digital field, which provides a track identifier (TID) number and additional information to identify the physical servo sector number. The TID is typically written in Gray code as the presence or absence of recorded dibits. During seek operations, when the head is moving across tracks, the head can typically only read a portion of the Gray-code in each TID. The Gray-code is constructed so that pieces of the TID, in effect, can be combined from adjacent tracks to give an approximate track location during a seek.
The SID field also supplies a position error field, which provides the fractional-track Position Error Signal (PES). Auxiliary functions, such as amplitude measurement control or repeatable run-out (RRO) fields are sometimes also used. During read or write operations the drive's servo control system uses the PES servo information recorded on the disk surface as feedback to maintain the head in a generally centered position over the target data track in order to read and write data on the track. The typical PES pattern includes a burst pattern in which the bursts are identical sets of high frequency magnetic flux transitions. Unlike the track-ID (TID) field number, the PES bursts do not encode numerical information. In contrast to the TID, it is the position of the bursts that provide information on where the head is relative to the centerline of a track.
FIG. 2A shows an SID 30A that supplies a position error field 24 with A & B bursts, which provides the fractional-track Position Error Signal (PES). Each track will have a large number of SIDs. The fields are arranged on the track so that the read head passes over the preamble first, i.e. the fields are read from left to right as illustrated. The typical PES patterns include either two or four bursts that are identical sets of high frequency magnetic flux transitions. The PES bursts are arranged in a pattern which generates a signal in the read head that is a function of the position of the read in relation to the centerline of the track. For example, the A and B bursts can be radially offset from each other by a half a track width and are sequential in the circumferential direction. Unlike the track-ID (TID) field number, the conventional PES bursts do not encode numerical information. The PES burst pattern is repeated for each set of two or four tracks, so only local information is provided.
Published U.S. patent application 20070279786 by Ehrlich, et al. (Dec. 6, 2007) describes a null-burst PES servo pattern with A, B, C and D bursts. The phase of the A burst is 180 degrees out of phase with the B burst. The A burst and the B burst are adjacent one another, and the border between them is on the centerline of a track. The phase of the C burst is 180 degrees out of phase with the D burst. The C burst and the D burst are adjacent one another and the border between them is on the edge of a track. When a read head is passing over the center of a track, the A burst and the B burst will generate a null or zero signal because the adjacent servo patterns will cancel out. When the read head is off center, the signal will have a varying amplitude and phase. The phase can be detected through a demodulation scheme. The amplitude can be detected through peak detection.
Each of these servo functions typically consumes a relatively independent portion of the servo wedge in prior art servo systems. The overhead on the disk to support these functions is a large factor in the drive's format efficiency. Typically, the servo fields can consume a significant portion of the recording surface of the disk and are an attractive target for reduction.
Integrated Servo concepts referenced herein are described in published U.S. patent applications: 20110149432, 20110149433, and 20110149434 by Coker, et al. (pub. Jun. 23, 2011). The Integrated Servo concept implements some or all major servo subfunctions for a storage device in Integrated Servo fields comprising sequences of encoded bits having selected mathematical properties. The Integrated Servo field is composed of a number of encoded sequences, which are members of a selected allowable sequence set that is constrained to provide some or all of the following functions: the Servo Track Mark (STM), the Position Error Signal (PES) and higher level positional information such as the track-ID. Thus, for example, an Integrated Servo embodiment would not need to have separate track ID fields using Gray code to encode the track ID. The integrated servo fields can provide a fractional Position Error Signal (PES) in relation to the center of a data track through the relative amplitude of the signal read for adjacent sequences disposed laterally across the tracks. The servo system detects the sequences in the signal from the read head using a set of digital filters corresponding to the set of encoded sequences. Embodiments of Integrated Servo constraint the placement of sequences so that only mathematically orthogonal sequences are placed next to each other on adjacent tracks.
FIGS. 2B-2C illustrate one prior art embodiment using Integrated Servo along other traditional fields in a hybrid design and the servo gate timing in hybrid and mini-mode. The SID 30B includes preamble 20C, a SAM 22, a track ID field 21A using Gray code, an Integrated Sequence Field 23, which includes first and second sequences 23A, 23B, and a repeatable run-out (RRO) field, which is an optional feature. FIG. 2B illustrates the timing of the standard servo gate window 25A, which allows all of the fields to be read. This mode is called the Hybrid-Servo mode herein. The prior art also includes using a reduced servo gate window 25B in which the assertion of the servo gate control signal is delayed until the end of the TID, which in this case is the start of Integrated Sequence Field 23. This allows the first and second sequences to be read and the information contained therein to be used. Modes in which the servo gate signal is delayed until the end of the TID and is shorter in duration will called “mini-mode” herein. The servo gate window 25B shown in FIG. 2C is one type of mini-mode. One use for the servo mini-mode is to allow the traditional write-to-read gaps between the preceding user data field and the following SID to be eliminated. The servo gate window 25B can be used in the case where the user data 26B has been written immediately before the SID and there is no write-to-read gap as illustrated in FIG. 2D. The write-to-read gap 27 has been included in all sectors in conventional designs to allow for the physical separation between the write head and the read head in the slider and to provide the time/distance needed to switch from writing data to reading the next servo sector ID (SID). The write-to-read gap makes the set of user data bits 26A smaller than the set of user data bits 26B because additional bits 28 can be written where the gap would otherwise be.
Augmented-servo-burst patterns in which information is encoded in addition to the fractional track PES have been described in the prior art. One example includes Gray code track ID fields plus diagonal burst PES with partial track ID information. See, for example, U.S. Pat. No. 7,110,209 to Ehrlich, et al. (Sep. 19, 2006).